The basic requirement of any laser is existence of population inversion between two atomic or molecular states in a gaseous or a solid material so that decay by stimulated emission may yield amplification. The production of population inversion needs pumping energy. In the case of conventional laser, the photon energies are eV order, and transition lifetime is nanosecond order. However, in the case of X-ray lasers the photon energies are hundreds or thousands eV order, and the transition life time is less than picosecond. This means that X-ray laser needs a large amount of pumping energy to be brought into the active medium in a very short time.
Much efforts have been done toward the development of x-ray and/or extreme ultraviolet (EUV) laser since the conventional lasers appeared in 1960. It was not until recently that soft x-ray lasers were successfully operated. The first operational laboratory X-ray laser is reported by Rosen et al in Physical Review Letters 54, 106 (1985), and experimentally demonstrated by Matthews et al in Physical Review Letters 54, 110 (1985). This seminal X-ray laser system is also fully disclosed by Campbell et al in U.S. patent application Ser. No. 676,338 filed Nov. 29, 1984. A related short wavelength laser, whose output extends into the EUV and X-ray region, is disclosed by Hagelstein in U.S. Pat. No. 4,589,113 issued May 13, 1986. However, those soft x-ray lasers are very large apparatus requiring very large energy lasers as the excitation source, typically delivering laser pulses of hundreds of Joules to kilojoules at longer wavelength. The cost, complexity, and size of the equipment required to form such x-ray laser apparatus make them impractical for most technical and scientific applications. Another disadvantage of these types of lasers is the low energy converting efficiency. This is because the energy converting process is from the electrical energy into long wavelength laser radiation, and then using the long wavelength laser to create a plasma from which the x-ray laser light emits.
Another approach for generating a x-ray laser is to directly deposit electrical energy in a plasma column which lases x-ray. These types of x-ray lasers are more direct and potentially more efficient than those driven by conventional laser drivers. Moreover, they use capacitors as energy sources to excite the plasma, and the size of the capacitor storing the same amount of energy as that of the laser driver output is much smaller than the size of the laser driver. There are two types of electrical discharge x-ray lasers: 1) Z-pinch discharge; 2) Capillary discharge. A power of 25 GW has been measured from 11 .ANG. spectral line in Z-pinch discharge by J. P. Apruzese, et al. in SPIE Proc. 875, 2 (1988). However, this Z-pinch x-ray laser is difficult to be combined with resonator for making large power, high coherent x-ray laser. In the capillary discharge type x-ray laser, a capillary tube is used to confine a column plasma which lases x-ray. The pulse duration in most of such capillary discharge type x-ray lasers tested to date is in the 100s of nanoseconds range. Because of interactions with the wall of the capillary on such time scales, the temperature of plasma has been limited to a few tens of eV. Therefore, the capillary discharge is difficult to be used for generating x-ray laser in a wavelength shorter than 100 .ANG..